I was doing some research on the sensitivity of the lateral line. This is a most amazing organ. One paper on the ability of a Toadfish to detect prey had information from numerous references, including this one in particular:
Montgomery and Bodznick (1994) indicated that the lateral line medullary nuclei contain an adaptive filter capability that cancels input consistently associated with an animal's own movements.
In other words, the fish ignores normal movements related to itself. Any movement up or down the water column would be irrelevant to the ability to detect external changes in the environment because the fish is hardwired to filter out it's own movements. I find this to be almost unbelievably sophisticated in otherwise primitive life forms. Sort of like the ability of Jellyfish to pursue prey, which many didn't believe possible either - until it was proven by controlled experimentation.
I grew up seeing barometric pressure expressed as inches, rather than millibars, so bear with me. The average pressure in my region is around 30 inches, and normal variation can take it up or down .1 to .3 inches in a 24 hour period. Major lows producing a drop of .5 inches - or roughly a 1.5-2 %
change - will result in spontaneous spawns from most small fish, such as Plant spawning Killifish, Corydoras Catfish, and Tetras. Note that most non-annual Killifish are also non-seasonal breeders, but a half inch drop in atmospheric pressure will regularly produce a ten fold increase in egg production in a 24 hour period. I have not observed a lot of Cichlids responding to these smaller, more common events. However, Apistogramma
, and Pelvicachromis
all have been observed to breed out of cycle
on these occasions.
The incident in which I had 4 diverse species of Cichlid spawn, at the same time that about 15 other species of fish responded to a drop in pressure, was very unusual. A drop of over 1 inch occurred - very rare in this region, as we are far from the oceans - within a 12 hour period. That is a change of only about 3.5 %.
But not only did I have a massive "fish Orgy", ALL the other large scale fish keepers in the local club also reported spawns from multiple species, including numerous Cichlids. Most of the cichlid keepers at that time had Malawian Mbuna and Haps, with a few Tanganikans. Whether or not any "Central American" species were involved I do not recall, other than my Geophagus steindachneri
. The only spawn I ever got in two years of keeping a species of unidentified Goby occurred that night.
I'm not claiming that the fish wait for such an event to spawn. But there has to be some tactical advantage to breeding under those conditions that would cause them to evolve the response. I believe it is instinctual, as many of them were simply unable to resist; even fish that weren't ready were triggered by a change in pressure of less than 4 %. Temperature did not change, as I live in Snow Belt Michigan, and the temperature inside my home is kept quite constant because outside it can change a lot in a short time. Light? the drop occurred at night, most spawning shortly after the timed lights came on. No water changes immediately prior. No special feedings. The only detectable environmental change was the drop in the barometer.
Personally, I find it amusing when people try to tell me that something I've observed hundreds of times doesn't happen. I can tell you empirically that it happens; what I can't tell you is why, or exactly how much of a change is required to trigger a response, or even whether or not all species have the same response. Maybe deepwater species don't need to worry about it, I don't know. I can't afford a hyperbaric (sp?) chamber to do the necessary controlled experiments. I would be greatly interested in hearing about it if someone actually does try.
There are two kinds of error: blind credulity and piecemeal criticism. Sound skepticism is the necessary condition for good discernment; but piecemeal criticism is an error. - Egyptian proverb